MITOSIS 49 



The whole process may be conceived as a play in four acts in which 

 there are no pauses between the acts. In the first act, the prophase, 

 the characters are introduced and several scenes are presented which lead 

 up to the second act, the metaphase. This is a grand tableau which 

 shows the stage fully set and the characters in formal array. In the 

 third act, the anaphase, a parade of the chromosomes results in their 

 separation and the division of all the characters into two groups at the 

 opposite sides of the stage. In the fourth act, the telophase, the char- 

 acters in each group adjust themselves to the changed conditions and 

 find their proper places in the new order. 



73. Significance of Mitosis. — The universality of this process in the 

 division of both animal and plant cells, and the regularity with which in 

 every case the various steps occur suggest that the process is of vital 

 importance. The great care with which the chromatin is divided between 

 the two cells seems to show that this division is, of all these steps, the 

 most significant. Recognizing in chromatin the substance which bears 

 the hereditary qualities from cell to cell, and in the case of sex cells from 

 animal to animal, this splitting has been conceived as having for its end 

 the passing on of the hereditary qualities to each of the two daughter 

 cells. Thus not only do these become structurally alike but each also 

 possesses the same inherited characteristics as the other and as the parent 

 cell. The various modifications of the process do not seem to affect this 

 judgment. The equal division of characteristics is explained on the 

 assumption that these characteristics correspond to units which are 

 arranged in a longitudinal series from one end of the chromatin thread to 

 the other, so that a longitudinal splitting of the thread involves the equal 

 division of every unit and therefore the sharing of every characteristic. 



74. Amitosis. — In contrast to the process just described, there has 

 always been recognized another type of cell division known as amitosis, 

 or direct cell division. Amitosis has been described as involving simply 

 the constriction of the cytoplasm into two portions, this constriction also 

 affecting the nucleus and dividing it into two portions, so that the whole 

 cell becomes divided into two parts containing equal amounts of cyto- 

 plasm and nucleus. It seems to occur only in cells which are highly 

 speciahzed, lacking in vitality, or undergoing degeneration. 



75. Continuity of Cell Life and Chromatin. — Two conceptions flow 

 directly from a consideration of the phenomena of cell division. One is 

 that all cells must be derived from previously existing cells, just as all 

 living things receive their life from previously existing living things. 

 This fact has been recognized for a long time and expressed in the apho- 

 rism omnis cellula e cellula, or "every cell from a cell," which we owe to 

 the German pathologist, Virchow. Another conception, based upon the 

 equal division of chromatin qualitatively and quantitatively between the 

 two daughter cells, is expressed in the phrase "continuity of chromatin" 



